Field of the Invention
The present invention relates in general to the transfer of data between a DSP in a modem and a host computer, and more particularly to a system and a method for pacing such transfer of data using a burst mode and a wait mode, to achieve optimal modem performance without impairing the overall operation of the host computer.
Data communication devices such as modems are important and ubiquitous devices that transmit and receive data. Data may include, for example, text, sound, images and video. In general, a modem operates by converting a digital data stream into a modulated analog signal for transmission over an analog telephone line. Because traditional analog telephone lines are designed to transmit analog information (such as voice), the modem ensures that the modulated analog signal conforms to the voice-like signal requirements of the analog telephone line. The modulation of digital data into an analog signal is typically done by a digital signal processor (DSP). The DSP also performs demodulation in which digital data is extracted from analog signals received over the telephone line. The DSP may also handle other tasks such as error detection and correction. In some cases, these other tasks can be performed by another device such as a microprocessor. Along with connecting to the telephone line, conventional modems also connect to a host computer, for example a personal computer (PC). Some type of hardware is required make this connection between the modem and the PC.
Modems can be divided into two categories based on how they are connected to a host computer, namely external modems and internal modems. In the most basic terms, this refers to whether or not the modem resides physically inside the host computer enclosure. External modems typically have their own enclosure and are connected to the host computer by a cable. At each end of this cable, there is usually a device called a universal asynchronous receiver-transmitter (UART). One UART is built into the host computer and a second UART is built in the modem. The first UART accepts information in a parallel format from the central processing unit (CPU) in the host computer. The UART rearranges this data into a serial format and transmits it across the cable to the modem. The UART in the modem converts the data back into a parallel format. The DSP (or other microprocessor) in the modem receives this data out of the UART and eventually the DSP modulates the data for transmission over the telephone line. Alternatively, data received from the telephone line is demodulated and sent to host computer in a similar manner using the same UARTs and cable.
Modems also can take the form of an add-in card that is installed in an expansion slot on the host computer. These can be classified as internal modems since they reside in the enclosure of the host computer. In this implementation, the modems typically connect directly to a parallel bus on the computer system. Using this parallel bus the CPU can transfer data to and from the modem without the need for rearranging the data into a serial format as is done with an external modem. Although the serialization is not required, many internal modems still include UART hardware without the serial elements. This hardware essentially replaces both UARTs and the cable used with an external modem as just described. As will be explained, this UART hardware is included so that internal type modems can be compatible with software written for use with an external modem.
A major factor in the design of personal computer systems is compatibility. Generally, new software should work with files created by older revisions of that software, new programs should work with existing hardware, and new hardware may have to work with existing software. Software running on the PC directs the modem operation and handles data movement in both directions. The first types of modems to become popular were external modems that employed a serial cable and a UART arrangement. Software that supported this hardware arrangement also became popular. Internal modems, which were subsequently developed, provided a UART interface to be compatible with the software that was written for external modems.
One important measure of modem performance is data transfer rate. The data transfer rate, also known as throughput, is the speed at which data can be transferred between devices. Data transfer rates are usually measured in kilobits (one thousand bits) or megabits (on million bits) per second. A higher data transfer rate means that more information can be transferred in a shorter period of time. A higher data transfer rate is desirable because network traffic, telephone toll changes, network access charges and download times are reduced.
Historically, data transfer rate was limited by the speed of the modulation techniques to send data over the analog telephone line. However, since modulation techniques have improved, the end-to-end speed of a modem connection is sometimes limited by the connection between the PC and the modem. For external modem UART hardware, the associated serialization of data causes this limitation. The inherent nature of this serialization paces the data so that data flows at a constant rate between the host computer and the modem. In the case of an internal modem, serialization is not necessary and UART hardware on these modems can support a much higher data rate. Unfortunately, the software that runs on the host or CPU expects to operate with a UART with serialization and the resultant data pacing. If data is allowed to flow through the UART without any type of pacing, the performance of the host computer can be adversely affected.
These performance problems can be understood by investigating in greater detail the operation of the host computer. Typically, the host computer includes software for communication over the modem. This software includes a special piece of code that is responsible buffering data and moving it in and out of the modem via the UART. One example of this type of code is an interrupt service routine (ISR.) In this case, the PC""s CPU might be running a variety of tasks, when an interrupt signal is received by the modem. This interrupt indicates that a certain number of bytes of data have been received over the serial interface and stored in the UART. In response to this interrupt, the CPU suspends its main tasks and starts the ISR code. The ISR code reads bytes from the UART until it is empty and then the CPU goes back to its main tasks. The ISR routine itself is relatively simple and short, but the CPU uses many extra instructions and time suspending and restarting its main task. This extra time is referred to as interrupt overhead.
In the case of a UART interface without serialization, it is possible that the DSP on the modem might be sending data to the DSP side of the UART as fast and the Host CPU can read them out. Despite the fact that the modem can only receive data from the telephone line at a limited rate, a large amount of data could be transmitted from the DSP to the host for each interrupt. This is possible because the DSP typically implements a large buffer in its memory and the entire buffer could be emptied at once. While this data is being moved, other tasks (including interrupts from other devices) are blocked. If other tasks are block for too long, the performance of the host computer is severely impacted. For instance, the movement of the mouse cursor across the monitor screen is noticeably slowed when the modem interrupt monopolizes the CPU on the host computer.
Notwithstanding the foregoing, it is possible it to achieve a high data transfer rate on an internal modem because there is no need to serialize the data. It is possible to increase the data transfer rate between a modem and a host computer by techniques not employing a UART, such as direct memory access or high speed serial connections. These approaches can increase data throughput and reduce the interrupt overhead. These approaches also require that compatibility to existing software be sacrificed.
Thus, there exists a need for a system and method of transferring data between a host computer and a modem that has an increased data transfer rate without seriously impairing (and perhaps improving) the performance of the host computer while remaining compatible with existing software.
The present invention increases data transfer rate and reduces interrupt latency while avoiding a concomitant increase in interrupts to the host by pacing the data flow between the UART and DSP using burst modes and wait modes. In order to pace the data transfer rate between the UART and DSP, a UART-to-DSP Interface (hereinafter xe2x80x9cUDIFxe2x80x9d) controls the flow of data between the UART and the DSP by alternatively bursting and halting data transfers.
During the burst mode, data is transferred across the interface at the maximum speed of the DSP (a xe2x80x9cburstxe2x80x9d) until the number of characters transferred reaches a predetermined burst count. During the wait mode, data transfer is suspended for the duration of a wait count. Data pacing control for the receive operation and for the transfer operation is independent.
The UDIF has at least one transmit channel and one receive channel and a plurality of registers for the implementation of the parallel to parallel UART to DSP mode. The registers contain information defining the number of character transfers of the burst mode and the duration of the wait mode.
These registers may be loaded by the DSP. Optimal wait and burst times for efficient data pacing may be selected. For example, a typical data rate used on modems such as 115.2 kilobits per second can be achieved by programming the registers. Optionally, the registers can be reprogrammed to double or quadruple that data rate without significant impact to the performance or the cost of the computer.